Method to induce strain in finFET channels from an adjacent region

ABSTRACT

Methods and structures for forming strained-channel finFETs are described. Fin structures for finFETs may be formed using two epitaxial layers of different lattice constants that are grown over a bulk substrate. A first thin, strained, epitaxial layer may be cut to form strain-relieved base structures for fins. The base structures may be constrained in a strained-relieved state. Fin structures may be epitaxially grown in a second layer over the base structures. The constrained base structures can cause higher amounts of strain to form in the epitaxially-grown fins than would occur for non-constrained base structures.

BACKGROUND

1. Technical Field

The technology relates to methods to induce strain in three-dimensionalmicrofabricated structures such as finFET structures. As an example, atype of strain (compressive or tensile) and amount of strain can beselectively induced in finFET channel structures from material andstructures formed adjacent to the channel regions.

2. Discussion of the Related Art

Transistors are fundamental device elements of modern digital processorsand memory devices, and have found numerous applications in variousareas of electronics, including power electronics. Currently, there area variety of transistor designs or types that may be used for differentapplications. Various transistor types include, for example, bipolarjunction transistors (BJT), junction field-effect transistors (JFET),metal-oxide-semiconductor field-effect transistors (MOSFET), verticalchannel or trench field-effect transistors, and superjunction ormulti-drain transistors. One type of transistor that has emerged withinthe MOSFET family of transistors is a fin field-effect transistor(finFET).

An example of a finFET 100 is depicted in the perspective view of FIG.1A. A finFET may be fabricated on a bulk semiconductor substrate 110,e.g., a silicon substrate, and comprise a fin-like structure 115 thatruns in a length direction along a surface of the substrate and extendsin a height direction normal to the substrate surface. The fin 115 mayhave a narrow width, e.g., less than 250 nanometers. There may be aninsulating layer 105, e.g., an oxide layer, on a surface of thesubstrate. The fin may pass through the insulating layer 105, but beattached to the semiconducting substrate 110 at a lower region of thefin. A gate structure comprising a conductive gate material 130 (e.g.,polysilicon) and gate insulator 135 (e.g., an oxide) may be formed overa region of the fin. Upper portions of the fin may be doped on eitherside of the gate structure to form a source region 120 and drain region140 adjacent to the gate.

FinFETs have favorable electrostatic properties for complimentary MOSscaling to smaller sizes. Because the fin is a three-dimensionalstructure, the transistor's channel can be formed on three surfaces ofthe fin, so that the finFET can exhibit a high current switchingcapability for a given surface area occupied on substrate. Since thechannel and device can be raised from the substrate surface, there canbe reduced electric field coupling between adjacent devices as comparedto conventional planer MOSFETs.

SUMMARY

The described technology relates to methods for making strainedmicrostructures, such as strained-channel finFETs, and to relatedstructures. According to some embodiments, a first straining layercomprising a first material may be deposited on a substrate in astrained state. The straining layer may be cut to relieve the strain andto form strain-inducing base structures. The base structures may besubsequently constrained on at least some of their exposed surfaces witha material having a high Young's modulus, so as to substantially lock inor freeze the strain-relieved state of the base structures. A secondmaterial having a lattice mismatch with the material of the basestructures may be epitaxially grown on the base structures. The secondmaterial may form in a strained state, and may be used, for example, toform channel regions of a finFET.

According to some embodiments, the straining layer may comprise acompound semiconductor (e.g., SiGe, SiC) deposited on a semiconductorsubstrate (e.g., Si). A second layer of material (e.g., Si) may beformed adjacent the straining layer, and a feature (e.g., a fin of afinFET) may be formed or patterned in the second layer. The adjacentsecond layer may be in direct physical contact with the straining layerin some embodiments, or may be separated from the straining layer by athin layer of material in some embodiments. The straining layer may bethin (e.g., between approximately 10 nm and 60 nm in some embodiments)such that strain in the straining layer is elastic rather than plastic,so that defects are not generated at unacceptable levels in thestraining layer or subsequent epitaxial layer formed over the straininglayer.

According to some embodiments, a method for making a strainedthree-dimensional feature (e.g., a fin of a finFET) on a substratecomprises forming a first semiconductor layer in a strained state at asurface of a substrate, and cutting the first semiconductor layer torelieve strain in the first semiconductor layer and to form at least onestrain-relieved structure. The method may further comprise depositing,after the cutting, a constraining material adjacent the strain-relievedstructure to restrict expansion and contraction of the strain-relievedstructure. The constraining material may have a Young's modulus higherin value than the Young's modulus of the strain-relieved structure.According to some embodiments, the method further comprises growing asecond semiconductor layer in a strained state adjacent to a surface ofthe constrained, strain-relieved structure, and forming the strainedthree-dimensional feature in the second semiconductor layer. The secondsemiconductor layer may have a lattice constant that differs from thatof the first semiconductor layer. When grown, e.g., by epitaxial growth,the second semiconductor layer may form in a strained state.

Structures related to the methods are also contemplated. In someembodiments, a strained-channel finFET structure formed on a substrateusing methods described herein may comprise a strain-inducing basestructure adjacent to a fin of the finFET structure, wherein thestrain-inducing base structure is formed from a first semiconductormaterial having a first lattice constant that is mismatched to a secondlattice constant of the fin material. The strained-channel finFET mayinclude a fin formed from a second semiconductor material that isstrained by the strain-inducing base structure, and further include aconstraining material adjacent the strain-inducing base structure. Theconstraining material may have a Young's modulus higher than the Young'smodulus of the strain-inducing base structure.

The foregoing and other aspects, embodiments, and features of thepresent teachings can be more fully understood from the followingdescription in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the figures, described herein,are for illustration purposes only. It is to be understood that in someinstances various aspects of the embodiments may be shown exaggerated orenlarged to facilitate an understanding of the embodiments. In thedrawings, like reference characters generally refer to like features,functionally similar and/or structurally similar elements throughout thevarious figures. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the teachings.Where the drawings relate to microfabrication of integrated devices,only one device may be shown of a large plurality of devices that may befabricated in parallel. The drawings are not intended to limit the scopeof the present teachings in any way.

FIG. 1A is a perspective view of a finFET formed on a bulk substrate;

FIG. 1B is a perspective view of a strained-channel finFET, according tosome embodiments;

FIGS. 2A-2L depict process steps and structures associated with methodsfor forming strained fin structures, according to some embodiments;

FIGS. 2M-2S depict alternative process steps and structures associatedwith methods for forming strained fin structures, according to someembodiments;

FIG. 2T depicts integration of n-MOS and p-MOS strained-channel, finFETdevices onto a same substrate, according to some embodiments;

FIG. 3 is a perspective view of a strained fin structure, according tosome embodiments;

FIGS. 4A-4C shows plots of longitudinal stress computed along a lengthof a strained silicon seed-layer structure, starting from a mid-point ofthe structure's length. The different plots represent results fordifferent heights of a SiGe strain-inducing base structure. Theconcentration of Ge in the strain-inducing base structure was 25%; and

FIGS. 5A-5C show plots of longitudinal stress computed at differentheights within a strained fin structure, for three different fin heights(30 nm, 20 nm, and 10 nm).

The features and advantages of the embodiments will become more apparentfrom the detailed description set forth below when taken in conjunctionwith the drawings.

DETAILED DESCRIPTION

As noted above, finFETs exhibit favorable current-to-size switchingcapabilities for integrated circuits, and finFETs like those shown inFIG. 1A have been fabricated in high density on bulk silicon (Si)substrates. FinFETs also exhibit favorable electrostatic properties forscaling to high-density, low-power, integrated circuits. Because the finand channel are raised from the substrate, the devices can exhibitreduced cross-coupling between proximal devices.

In some cases, speed, junction leakage current, and/or breakdown voltageconsiderations may create a need for semiconductor material other thansilicon. For example, SiGe can exhibit higher mobilities for electronsand holes, higher device speed, and lower junction leakage than bulk Si.As a result, some devices may be fabricated from SiGe that isepitaxially grown on a silicon substrate. However, conventionalepitaxial growth of SiGe for forming integrated devices has someattributes that may not be favorable for certain applications. Forexample, because of a lattice constant mismatch between SiGe and Si,strain is induced in the SiGe as it is epitaxially grown. In some cases,the strain causes defects such as dislocations to form in the SiGeduring its growth, which can adversely affect device performance anddegrade performance to unacceptable levels. To mitigate effects ofstrain, a thick epitaxial layer of SiGe may be grown so that the strainis relieved over an appreciable distance. Depending upon the dopantconcentration, SiGe epitaxial layers 1-10 microns thick may be necessaryto relieve the stress by full plastic relaxation. Plastic relaxation canintroduce defects. Such an approach may require long and complex epitaxysteps (e.g., it may be necessary to vary dopant concentration during theepitaxial growth), and may further require a thermal annealing step andchemical-mechanical polishing step to planarize a surface of the SiGeafter its growth. The annealing may reduce some defects generated duringepitaxial growth of SiGe, but typically the defects may not be reducedbelow 10⁵ defects/cm², a level not suitable for many industrialapplications.

Straining of silicon can be used to improve some of its electricalproperties. For example compressive straining of silicon can improve thehole mobility within silicon. Tensile straining of Si can improveelectron mobility. The inventors have recognized and appreciated thatinducing strain in silicon can enable fabrication integrated electronicdevices based on Si with performance that is competitive with SiGedevices. Although a SiGe compressive fin may exhibit higher holemobility as compared with a Si compressive fin, the Si compressive finis free of alloy scattering, an effect that can adversely affect holetransport in SiGe fins. Accordingly, a compressively-strained, Si finFETmay be competitive with a compressively-strained SiGe finFET in terms ofperformance, and fabrication of compressive Si fins may be easier andmore cost effective than fabrication of compressive SiGe fins. Forsmaller devices, e.g., sub-20-nm channel-width FETs such as finFETs, theavoidance of thick (>1 micron) SiGe epitaxial layers and increaseddevice performance from strain may be important factors in themanufacturability of the strained devices.

A strained-channel finFET 102 may appear as depicted in FIG. 1B,according to some embodiments. The structure may be similar to thatshown in FIG. 1A except that a portion of the fin structure 215 includesa strain-inducing base structure 222. The strain-inducing base structuremay comprise a material or alloy different than the material or alloy ofthe fin 212 where the device's active region is located. For example, ina Si-based device, the fin 212 may be formed from bulk and/or epitaxialsilicon. The strain-inducing base structure 222 may be formed from SiGein some embodiments (e.g., to induce tensile stress in the fin andimprove electron mobility in the device), or SiC in other embodiments(e.g., to induce compressive stress in the fin and improve hole mobilityin the device. Constraining structures 242 may be located adjacent tothe strain-inducing base structure 222. As may be appreciated, othermaterials exhibiting a lattice mismatch with the substrate and devicelayer may be used instead of SiGe or SiC, and different material systemsmay be used in other implementations. In some implementations, the finstructure 215 may be formed and an insulating material subsequentlyformed between the fin structure and substrate 110, using techniquesdescribed in U.S. patent application Ser. No. 13/964,009 titled, “BULKFINFET SEMICONDUCTOR-ON-NOTHING INTEGRATION,” and filed Aug. 9, 2013,which application is incorporated by reference in its entirety.

FIGS. 2A-2L depict process steps that may be used to fabricatestrained-channel finFET devices beginning with a bulk semiconductorsubstrate. In overview, the active regions of the fins may be formedfrom epitaxially-grown semiconductor of high quality (e.g., epitaxial Siin the example). The fins may be epitaxially grown on a strain-inducingbase structure 222 of a second material type, which may be epitaxiallygrown on a substrate 110. The base structures 222 may be formed from astraining layer 220 which, because of a lattice mismatch with thesubstrate, epitaxially grows in a strained state. When a base structure222 is cut or formed in the straining layer 220, the strain will berelieved locally at the base structure. The strain-relieved basestructure can be constrained in this state by forming a material 242with a high Young's modulus to partially encapsulate the base structure.Subsequent epitaxial growth and formation of a fin 212, of a differentmaterial than the base structure, on the constrained base structure canimpart stress on the order of 10⁹ Pascals or more to an active region ofthe fin. Since the formation of the strain-inducing base structure, itsrelaxation, and subsequent formation of the fin may be in a purelyelastic regime, there may be no appreciable defects generated in thestrain-inducing base structure 222 and/or the epitaxially grown fin 212,as would be generated from thicker layers that may suffer from plasticdeformation and relaxation, for example. Further details regardingdevice fabrication are described below.

According to some embodiments, a process for forming a strained-channelfinFET may begin with a substrate 110 of a first semiconductor material,as depicted in FIG. 2A. The substrate may be a semiconducting substrate(e.g., a bulk Si substrate), though other semiconducting materials maybe used in other embodiments. In some embodiments, the substrate maycomprise a silicon-on-insulator (SOI) substrate. The substrate may be ofp-type conductivity or of n-type conductivity.

With reference to FIG. 2B, at a region where fins for finFETs are to beformed, additional layers may be formed on the substrate surface. Forexample, a straining layer 220 may be formed over the substrate 110. Insome embodiments, for a Si substrate, a SiGe straining layer 220 may beepitaxially grown on the substrate 110. A thin buffer or seed layer 210(e.g., a thin Si layer) may be epitaxially grown on the straining layer220, and a “soft” layer 205 may be formed over the buffer layer 210. Invarious embodiments, there is a lattice mismatch between the straininglayer and the substrate, so that the straining layer forms withcompressive or tensile, in-plane stress. Further there may be a latticemismatch between the straining layer 220 and the seed layer 210. In someembodiments, the seed layer may be omitted.

According to some embodiments, the straining layer 220 comprises SiGeand may have a thickness between about 10 nm and about 60 nm. In someembodiments, the straining layer 220 may be a “full sheet” layer (e.g.,extending uniformly across the entire surface of the substrate 110). Thepercentage concentration of Ge in the straining layer may be betweenabout 1% and about 50%. The concentration of Ge and the thickness of thestraining layer may be selected to impart a desired amount of strain toa structure formed over the straining layer. In some implementations,the dopant concentration of Ge in the straining layer may depend upon athickness of the straining layer. For example, a higher dopantconcentration may be used in thinner layers. The dopant concentrationmay be selected such that defect densities generated by the strain arebelow a desired value. According to some embodiments, a straining layerthat is approximately 30 nm thick may have a Ge concentration of about30%, whereas a straining layer that is approximately 40 nm thick mayhave a Ge concentration of about 25%. In some implementations, astraining layer may have a thickness up to about 60 nm with a Geconcentration of about 25%, without exceeding acceptable defect levels.The straining layer 220 may be formed by chemical vapor deposition,atomic layer deposition, or any other suitable crystal-growth process.

In some embodiments, the straining layer may comprise SiC, and may havea thickness between about 5 nm and about 60 nm. The percentageconcentration of C in the straining layer may be between about 1% andabout 50%. Other materials may be used for the straining layer 220 inother embodiments.

The terms “approximately” and “about” may be used to mean within ±20% ofa target dimension in some embodiments, within ±10% of a targetdimension in some embodiments, within ±5% of a target dimension in someembodiments, and yet within ±2% of a target dimension in someembodiments. The terms “approximately” and “about” may include thetarget dimension.

In various embodiments, the straining layer is formed by epitaxialgrowth, so as to form a crystalline layer with low defect concentration.By limiting the alloy concentration and thickness of the straining layer220, the layer may form in a strained state in which all strain ispurely elastic. As such, the strain may induce few defects in thestraining layer. For example, the straining layer may have a defectdensity less than 10⁵ defects/cm² in some embodiments, less than 10⁴defects/cm² in some embodiments, less than 10³ defects/cm² in someembodiments, less than 10² defects/cm² in some embodiments, and yet lessthan less than 10 defects/cm² in some embodiments. By controlling theepitaxial growth conditions for the straining layer 220, the crystallinequality of a subsequently grown seed layer 210 or device layer in whichan active structure is formed may be high with as few or fewer defectsthan the straining layer.

In some embodiments, a processing thermal budget may be controlled afterformation of the straining layer, so as to avoid unwanted mechanicalrelaxation of the straining layer and formation of dislocations in thestraining layer, and to avoid diffusion of the dopant (Ge or C, forexample) of the straining layer to the device region (e.g., into thechannel region of the finFET). For example, a 1000° C., 30-minute bakemay be unacceptable thermal processing, whereas a spike anneal to atemperature of 1040° C. at a ramping rate of 75° C./sec with no plateauwould be sufficient to activate the dopant without unacceptable adverseeffects of dislocations or dopant diffusion.

According to some embodiments, the seed layer 210 forms a layer uponwhich fins for strained-channel finFETs may be formed. In someembodiments, the semiconductor material of the seed layer 210 may bedifferent than the semiconductor material of the substrate. In otherembodiments, the semiconductor material of the seed layer 210 may be thesame as the semiconductor material of the substrate. The thickness ofthe seed layer may be between about 1 nm and about 10 nm in someembodiments. The seed layer 210 may be formed by chemical vapordeposition, atomic layer deposition, or a suitable crystal-growthprocess.

The soft layer 205 may comprise a material that has a Young's modulusless than that for the straining layer 220, e.g., less than about 20 GPain some embodiments, less than about 10 GPa in some embodiments, and yetless than about 5 GPa in some embodiments. In some implementations, thesoft layer may comprise an oxide, e.g., SiO₂. The Young's modulus of thesoft layer may be less than the Young's modulus of the straining layerby a factor of more than 2 in some embodiments, by a factor of more than4 in some embodiments, by a factor of more than 8 in some embodiments,an yet by a factor of more than 20 in some embodiments. The thickness ofthe soft layer 205 may be between about 5 nm and about 100 nm. In someembodiments, the soft layer may exhibit etch selectivity over the seedor buffer layer 210 and the straining layer 220.

Strain-inducing base structures may be patterned in the straining layer220 by a sidewall image transfer (SIT) process that is depicted by stepsillustrated in FIGS. 2C-2E. For this process, additional layers may bedeposited and patterned over the epitaxial layers and soft layer, asindicated in FIG. 2C. Bar-like structures 252 may be formed in apatterning layer deposited over the soft layer using any suitablemethod, e.g., photolithography and etching. The photolithography mayrequire forming a photoresist layer over the patterning layer, exposingand developing the photoresist, and etching the patterning layer to formthe bar structures 252. In some embodiments, the bar-like structures maybe patterned using a mandrel lithography process. In someimplementations, the bar-like structures may be patterned usinginterferometric lithography techniques. The bar-like structures 252 maybe patterned to extend for a length L (into the page) that isapproximately a desired length for a fin of a finFET transistor. Thewidth W and spacing S of the bar-like structures may be chosen toprovide desired spacings between multiple fins of a finFET device orbetween multiple finFET devices.

A blanket layer (not shown) may be deposited over the bar-likestructures 252 and soft layer 205. In some embodiments, the blanketlayer may comprise silicon nitride that is deposited by a plasmadeposition process. The thickness of the blanket layer may be between 50nm and 100 nm in some embodiments, between 5 nm and 50 nm in someembodiments, and in some embodiments may be between about 5 nm and about20 nm. The blanket layer may form conformally on the sidewalls of thebar structures 252. The blanket layer may be etched away from planarsurfaces and partially etched on the vertical surfaces to form spacerstructures 232, as depicted in FIG. 2C. The thickness of the blanketlayer may then determine approximately a width w_(s) of the spacerstructures 232, and subsequent widths of fin structures. A series ofetching steps may then be used to pattern the strain-inducing basestructures in the straining layer 220, where the spacer structures 232substantially define the pattern of the strain-inducing base structures.

For example, a first selective, anisotropic etch may be performed toremove the bar-like structures 252. The same etch recipe, or a differentetch recipe may be used to remove most of the soft layer 205, therebytransferring the pattern of the spacer structures 232 to the soft layer.The resulting structure may appear as depicted in FIG. 2D. A secondselective, anisotropic etch may be performed to remove exposed portionsof the seed layer 210 and straining layer 220, thereby transferring thepattern from the soft layer to the seed layer and straining layer so asto form the strain-inducing base structures 222. In some embodiments,the spacer structures 232 may etch away during etching of the seed layerand straining layer to yield a structure like that depicted in FIG. 2E.In some implementations, the spacer structures may be removed by adedicated etch.

In some embodiments, there may be additional etching into the substrate110, as depicted in FIG. 2E. For example, the etching may extend between5 nm and 50 nm into the substrate 110. By etching into the substrate, asmall pedestal 111 under the strain-inducing base structure 222 may beformed. This pedestal may facilitate strain relief in the base structure222, and increase an amount of strain developed in a subsequently-grownfin structure. In some implementations, the etching may stop atapproximately the original surface of the substrate 110.

As noted above, the straining layer 220 forms in a strained state duringits epitaxial growth due to a lattice mismatch between the material usedfor the straining layer and the substrate. As an example, a SiGestraining layer will form with compressive strain when grown on a bulkSi substrate. The amount of strain in the SiGe layer can be controlledby controlling the Si:Ge ratio and controlling the thickness of thestraining layer. The etching to form the base structures 222 andpedestal 111, the soft layer material, and, in some cases, removal ofthe spacer structures 232 allows the base structures to relax so as torelieve strain. Because the soft layer 205 has a lower Young's modulus,most of the strain in the base structures is relieved. In someembodiments, the release of strain in the base structures may be purelyelastic, such that no appreciable defects are generated. Because thebase structures may be narrow and long, the release of strain issubstantially uniaxial (e.g., longitudinal along the length of the basestructure 222).

A constraining layer 240 may be deposited over the base structures 222,as depicted in FIG. 2F, so as to partially encapsulate and constrain thebase structures in the strain-relieved state. The constraining layer 240may have a Young's modulus that is greater than 50 Gpa in someembodiments, greater than 100 Gpa in some embodiments, and yet greaterthan 200 Gpa in some embodiments. According to some embodiments, theconstraining layer comprises a nitride, e.g., Si₃N₄. The constraininglayer may be deposited using any suitable conformal deposition process.The thickness of the constraining layer may be between about one-halfthe width w_(s) of the base structure and about twice the width of thebase structure 222. According to some embodiments, the thickness of theconstraining layer is about equal to the width w_(s) of the basestructure, so as to provide sufficient rigidity to hold the basestructure 222 in a stress-relieved state. By holding the base structure222 in a stress-relieved state, a majority of stress is developed in afin structure subsequently grown on the base structure.

The constraining layer 240 may then be etched using an anisotropic etch,so as to form constraining structures 242 adjacent the strain-inducingbase structures 222, as illustrated in FIG. 2G. The anisotropic etch mayremove portions of the constraining layer 240 over the base structures222, so as to expose cap portions 204 of the soft layer, and may removeportions extending across a surface of the substrate adjacent to thebase structures.

An insulating layer 207 may then be deposited over the base structuresand constraining structures, as depicted in FIG. 2H. The insulatinglayer may be deposited by any suitable process, and in some embodimentsmay be a spin-on glass that is subsequently baked. The insulating layer207 may be etched back (using a chemical-mechanical polishing (CMP)step, for example) so as to planarize the upper surface 209 and exposethe remaining cap portions 204 of the soft layer, as depicted in FIG.2I. The removal of the insulating layer 207 may stop on the constrainingstructures 242, and may remove a portion of the constraining structures.

The cap portions 204 of the soft layer, and insulating layer 207 in somecases, may be etched using an anisotropic etch to expose the seed layer210, as depicted in FIG. 2J, though other etching techniques may beused. In some embodiments, isotropic etching may be used. In someimplementations, a wet etch may be used to expose the seed layer 210.The etch rates of the cap portions 204 of the soft layer and theinsulating layer 207 may be different.

After exposure of the seed layer, fins 212 may be epitaxially grown asdepicted in FIG. 2K. Since the strain-inducing base structures 222 havebeen substantially constrained by the constraining structures 242, theycannot absorb appreciable strain as the fins 212 grow. As a consequence,the fins 212 receive most of the strain due to the lattice mismatchbetween the base structure material and the fin material, and the amountof strain imparted to the fins 212 can be a factor of two higher thanwould be imparted if the base structures 222 were not constrained andwere allowed to absorb some of the strain.

According to some embodiments, the constraining structures 242 may beetched back, as depicted in FIG. 2L, so as to expose the fin 212 forsubsequent processing that will produce a finFET as depicted in FIG. 1B.According to some embodiments, the etch back of the constrainingstructures 242 may reduce the tops of the constraining structures to alevel approximately equal with the base of the fins 212, although thetops of the constraining structures may be slightly above or below thebase of the fin. The insulating material 207 may or may not be etchedback further.

FIGS. 2M-2S depict an alternative embodiment of a process forfabricating strained fin structures. The embodiment shown in thesefigures may depart from the previously-described embodiment afterforming a structure depicted in FIG. 2G. According to some embodiments,a second nitride layer 247 may be formed as depicted in FIG. 2M over theconstraining structures 242 and base structures 222 of FIG. 2G. Thesecond nitride layer may be formed using a vapor deposition process orany suitable conformal deposition process. In some embodiments, amaterial other than nitride may be used (e.g., a material exhibitingetch selectivity over oxide). An insulating material 206 (e.g., SiO₂)may be deposited on the substrate and planarized, as illustrated in FIG.2N. The insulating material may exhibit etch selectivity over thenitride layer 247. Planarization may be done via chemical-mechanicalpolishing, and may stop on the nitride layer 247, according to someembodiments.

Subsequent the planarization, the insulating material 206 may be etchedback using a anisotropic etching process (e.g., a SiCoNi etching step orreactive-ion etching step). The etch may reduce the height of theinsulating material 206 to a level below the top surface of the capportions 204, as depicted in FIG. 2O. A hard mask material 249 (e.g., anitride) the exhibits etch selectivity over the insulating material 206may then be deposited, as depicted in FIG. 2O. The mask material 249 maybe deposited by any suitable deposition process, e.g., evaporation,plasma deposition, vapor phase deposition, or sputtering.

The hard mask material 249 may be planarized, e.g., via a CMP process,stopping on the cap portions 204, as depicted in FIG. 2P. Some of thecap portions 204 may be removed during the CMP step. A portion of thehard mask material 249 may remain as masking features 248 over theinsulating material 206, whereas an upper surface 208 of the capportions may be exposed for a subsequent etch. The cap portions 204 maythen be etched away (e.g., using reactive ion etching or a SiCoNietching process) to expose the underlying seed layer 210, as depicted inFIG. 2Q.

Fin structures 212 may then be epitaxially grown on the seed layers asillustrated in FIG. 2R. The constraining structures 242 provide a formthat substantially determines the shape of the fin structures. The finstructures may be grown to a selected height. The substrate may then besubjected to etching to remove a portion of the hard mask material 247and a portion of the insulating material 206, so as to expose the finstructures 212, as depicted in FIG. 2S. In some embodiments, a wet etch(e.g., an wet etch in heated phosphoric acid) may be used to remove aportion of the hard mask material 247 and a portion of the constrainingstructures 242. The wet etch may be a timed etch, stopping atapproximately a base or bottom of the fin structure 212. In someembodiments, the wet etch may stop above the base of the fin structure.In another embodiment, the wet etch may stop below the base of the finstructure 212. The insulating material 206 may then be etched back toapproximately the base of the fin structure, e.g., using a SiCoNiprocess or other suitable reactive-ion etching process.

The process steps described in connection with FIGS. 2M-2S yield astructure different from an alternative method described in connectionwith FIGS. 2H-2L. For the steps shown in FIGS. 2M-2S, there is aninterfacial hard mask layer between the substrate 110 and the remaininginsulating material 206. This interfacial layer is not present in thealternative process, and its presence or absence may distinguish betweenthe two processes. The steps associated with FIGS. 2M-2S may provideadditional mechanical support to the fin structures, and allowadditional cleaning (e.g., via HF etching) of the seed layer prior toepitaxial growth of the fin.

Although the acts described in FIGS. 2A-2S referred primarily to aSi/SiGe combination of materials, other combinations are possible. Forexample, in some embodiments, Si and SiC may be used. The SiC may beused to form the strain-inducing base structures 262 for p-type finFETs,as depicted in FIG. 2T. Additionally, both types of devices may beformed on a same substrate 110. The structures may be used to form CMOSfinFETs, e.g., p-type devices using Si/SiC material combinations andn-type devices using Si/SiGe material combinations. The complementarydevices may be formed at substantially a same device level on asubstrate, or at different levels.

To investigate the amount of strain imparted to fins, numericalsimulations utilizing finite element analysis were carried out. Thesecomputations show that strain in excess of 1 GPa can be imparted tochannel regions of finFETs using the above-described fabricationtechniques. Among the controlling parameters are lattice mismatchbetween the straining layer and substrate (controllable through choiceof materials and/or dopant concentrations), thickness or height of thestraining layer, thickness of the constraining layer and its Young'smodulus, thickness or height of the fin, and length of the fin.

FIG. 3 shows a perspective view of a fin structure in which longitudinalstress was numerically analyzed. For purposes of viewing the structure,one of the constraining structures 242 is omitted from the drawing, butwas included in the simulations. The longitudinal stress S_(yy) wascomputed along a length L of the fin from its center, and at variousheights (in the direction of H) at the fin center. According to someembodiments, an active region or channel of a finFET would be located atthe center of the fin 212, e.g., at the region between the dashed linesin FIG. 3.

Two sets of simulations were run. In a first set, only the seed layer210 was present above the strain-inducing base structure. The resultsfrom these simulations are shown in FIGS. 4A-4C. In the second set ofsimulations, a silicon fin 212 was regrown on the seed layer, and stressplots were generated for different heights within the fin. Threedifferent fin heights were trialed in the second set of simulations. Theresults from these simulations are shown in FIGS. 5A-5C. In both sets,the thickness or height of the strain-inducing base structure wasselected to be 40 nm, and the concentration of Ge in the SiGe basestructure was selected to be about 25%. This concentration of Geintroduced about 1% strain in the SiGe layer. The length of thestrain-inducing base structure 222 and fin 212 was selected to be about128 nm for both sets of simulations. The transverse width of a fin wasselected to be about 10 nm, and the thicknesses of the adjacentconstraining structures were selected to be about 10 nm. Theconstraining material was selected to be Si₃N₄. The values selected forthe simulations were chosen only for purposes of illustration, and arenot meant to be limiting.

In the first set of simulations (FIGS. 4A-4C), the strain induced in thebuffer layer 210 is found to be greater than 1.2 GPa at the midpoint (interms of length) of the seed layer 210, for a strain-inducing basestructure that is about 40 nm in height (FIG. 4A). The strain istensile, indicating that the underlying SiGe, formed under compressivestress, has relaxed so as to place the Si buffer layer 210 under tensilestress. The strain increases moving toward an end of the fin, and thendrops sharply within about 20 nm from the end of the fin. FIGS. 4B and4C show that the strain at the center of the seed layer (a region wherethe channel of the finFET may be formed) falls to about 0.6 GPa when theheight of the strain-inducing base structure 222 is reduced to about 20nm.

Interestingly, for FIGS. 4B and 4C, there remain regions along the seedlayer 210 where the induced strain remains above about 1 GPa. In someembodiments, channel regions of two separate finFETs may be formed atthese high-strain regions on a single fin structure like that shown inFIG. 3. The two finFETs may share source connections at the center ofthe fin, for example, and separate drains may be located at opposingends of the fin.

For the results shown in FIGS. 5A-5C, the height of the strain-inducingbase structure 222 was fixed at about 40 nm. Other parameters of thebase structure 222 were the same as used for FIGS. 4A-4C. The height ofthe regrown fins were selected to be 30 nm (FIG. 5A), 20 nm (FIG. 5B),and 10 nm (FIG. 5C). For each graph, several simulation results areprovided for different height locations within the fin 212 (uppertraces). The traces with the highest strain values reflect an amount ofstrain near the base of the fin 212, e.g., near the interface betweenthe strain-inducing base structure 222 and fin 212. The tracesimmediately below the top trace reflect strain values at increasingheights in the fin.

FIG. 5A shows that for a 30-nm thick fin, the induced strain is about1.5 GPa near the base and center of the fin, and that the induced strainreduces to about 0.6 GPa near the top of the fin. Again, the inducedstrain falls in value toward the end of the fin structure, but fallsmore gradually than for the structures simulated in FIGS. 4A-4C. Thelower trace in FIG. 5A shows strain just inside the base structure.

FIGS. 5B-5C show that the strain increases, to more than 2.5 GPa, whenthe thickness of the fin reduces to about 10 nm (e.g., a nanowiredimension). As the figures show, high values of stress can be achievedfor very thin layers of semiconductor materials. Further, because thelayers are thin, e.g., less than about 60 nm in some embodiments, theremay be no significant defect formation, since the materials are strainedin the elastic regime. Accordingly, high-performance Si devices that maybe competitive with alloy devices (e.g., SiGe or SiC devices) may befabricated using the techniques described above.

Although the results shown in FIGS. 4A-4C and FIGS. 5A-5C were obtainedfrom samples with uniform doping (25% Ge) in the straining layer 220, insome embodiments, it may be possible to increase the Ge content (or Ccontent for a SiC straining layer) and/or increase the thickness of thestraining layer beyond the amounts described before creating anunacceptable defect density. In some embodiments, a gradient in Ge (orC) content may be used to increase the thickness of the straining layerwithout creating an unacceptable defect density. The gradient in Ge (orC) may be in the direction of epitaxial growth, e.g., in a directionperpendicular to an interfacial surface between the straining layer 220and the buffer layer 210. In some embodiments, the Ge (or C) content maybe between about 10% and about 25% within the straining layer. In someembodiments, the Ge (or C) content may be between about 25% and about50% within the straining layer.

The fins shown in the drawings may be spaced laterally from each otheron one or more regular spacing intervals. For example, there may be auniform lateral spacing d₁ between all fins. Alternatively, there may betwo uniform lateral spacings d₁, d₂ alternating between successive fins.The fins may have a width between approximately 5 nm and approximately30 nm. The fins may be spaced apart between approximately 10 nm andapproximately 50 nm, in some embodiments. In some embodiments, the finsmay be spaced apart between approximately 50 nm and approximately 250nm. According to some embodiments, the fin spacing or pitch may bebetween about 40 nm and about 60 nm. There may be one or more fins perfinFET device. A gate structure, like that shown in FIG. 1B may beformed over one or more strained fins to form a finFET. For example,there may be a common gate shared by multiple strained fins.

A finFET device fabricated according to the present teachings may beformed in an integrated circuit in large numbers and/or at highdensities. The circuits may be used for various low-power applications,including but not limited to, circuits for operating smart phones,computers, tablets, PDA's, video displays, and other consumerelectronics. For example, a plurality of finFETs fabricated inaccordance with the disclosed embodiments may be incorporated inprocessor or control circuitry used to operate one of the aforementioneddevices.

The discussion above is directed primarily to a SiGe straining layerthat imparts tensile stress to a fin of a Si finFET device. Accordingly,for a Si finFET, the use of SiGe for the straining layer may improve theelectron mobility for n-channel finFETs. For p-channel finFETs, SiC maybe used as the straining layer. SiC can impart compressive stress to afin. In alternative embodiments for which an active fin and channel maybe formed in SiGe, the materials may be reversed. For example, Si may beepitaxially grown on a SiGe substrate or base layer to form a straininglayer of Si. The buffer layer and fin may then be formed of SiGe.

Although embodiments described above are directed to fabrication ofstrained-channel Si finFET devices, the methods of inducing strain maybe extended to other devices or structures, in which other materials maybe used. The techniques may be applied to other types of finFETs, e.g.,fully insulated finFETs such as silicon-on-nothing finFETs, and othermicrofabricated devices and structures such as MEMs devices. In someembodiments, the techniques may be used in LEDs or laser diodes tostrain the device and adjust emission wavelength. According to someembodiments, a straining layer and strain-inducing structure may be usedto impart stress to any three-dimensional structure patterned into adevice layer that has been formed adjacent the strain-inducingstructure. For example, a three-dimensional device or structure may beformed adjacent a strain-inducing structure that is shaped for theparticular device so as to impart strain to the three-dimensional deviceor structure.

In some embodiments, the straining layer and/or seed layer may comprisea material other than semiconductor material, e.g., crystallineinsulator, an oxide, a ceramic, etc. In some embodiments, the straininglayer and/or seed layer may be formed by methods other than epitaxialgrowth, e.g., plasma deposition, plasma deposition and annealing,sputtering, etc. followed by an anneal. According to some embodiments, athin insulating layer, or a layer of a different material, may be formedbetween the straining layer and the seed layer. In some embodiments, atleast a portion of the straining layer may comprise an active region orportion of a formed device.

The technology described herein may be embodied as a method, of which atleast one example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.Additionally, a method may include more acts than those illustrated, insome embodiments, and fewer acts than those illustrated in otherembodiments.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. A finFET comprising: a substrate; astrain-inducing base structure vertically above the substrate, thestrain-inducing base structure comprising a first semiconductor materialhaving a first lattice constant; a constraining material laterallyadjacent the strain-inducing base structure, the constraining materialhaving a Young's modulus higher than a Young's modulus of the straininducing base structure; and a fin vertically above the strain-inducingbase structure and comprising a second semiconductor material having asecond lattice constant that is different than the first latticeconstant.
 2. The finFET structure of claim 1, wherein the firstsemiconductor material comprises SiGe or SiC.
 3. The finFET structure ofclaim 2, wherein a Ge or C content of the SiGe or SiC is betweenapproximately 10% and approximately 25%.
 4. The finFET structure ofclaim 2, wherein a Ge or C content of the SiGe or SiC is betweenapproximately 25% and approximately 40%.
 5. The finFET structure ofclaim 2, wherein the first semiconductor material has a gradient in Geor C concentration in a direction perpendicular to an interfacialsurface between the strain-inducing feature and the fin.
 6. The finFETstructure of claim 1, wherein the second semiconductor materialcomprises Si.
 7. The finFET structure of claim 1, wherein a thickness ofthe first semiconductor material is between approximately 10 nm andapproximately 60 nm.
 8. The finFET structure of claim 1, wherein athickness of the second semiconductor material is between approximately10 nm and approximately 60 nm.
 9. The finFET structure of claim 1,wherein the fin has a width between approximately 5 nm and approximately30 nm.
 10. The finFET structure of claim 9, further comprising a gatestructure formed at a center region of the fin.
 11. The finFET structureof claim 10 disposed in a smart phone, computer, tablet computer, PDA,or video display.
 12. The finFET structure of claim 1, wherein theYoung's modulus of the constraining material is at least twice the valueof the Young's modulus of the strain-inducing base structure.